This invention relates to a velocity control device which provides relatively free rapid movement of a member in a fluid medium in one direction while providing a relatively high resistance to rapid movement of the member in the opposite direction.
The invention is useful, for example, with the selectively insertable control rods of a nuclear reactor whereby the control device allows rapid movement of the control rods into the nuclear core for rapid reactor shutdown but limits the velocity of movement of the control rods out of the core to thereby limit the rate of increase of reactivity of the core.
Previous velocity limiter devices for such purposes are shown, for example, by Trocki et al in U.S. Pat. No. 3,395,781 and by McClintic et al in U.S. Pat. No. 3,397,759 which are incorporated herein by reference. In particular the embodiment shown in FIG. 7 of McClintic et al has been used commercially long and successfully. This embodiment uses a conical velocity control member attached to the drive hub at the lower end of a control rod which, in cooperation with an annular skirt member, increases fluid turbulence and hence provides resistance to rapid downward motion of the control rod.
A drive mechanism for selectively inserting the control rods into the core is shown, for example, by R. R. Hobson et al in U.S. Pat. No. 3,020,887.
Prior control rods typically employed boron carbide, in the form of powder contained in tubes, as the neutron absorbing material.
More recently the desire for increased control rod life has resulted in the use of neutron absorber material of higher density and weight such as hafnium metal.
Since, for bottom entry control rods, the weight of the control rod constitutes at least a part of the force tending to drive the control rod downward and out of the core, a velocity limiter of greater effectiveness, to offset the greater downward force of the control rod, is desirable.
Furthermore, it is desirable that the weight of the velocity control device itself be reduced to thereby minimize the overall increase in the weight of the control rod. Minimizing weight is an important factor since the scram time (the time needed to insert a control rod into the core) is a function of the overall weight of the control rod.
Therefore, an object of the invention is a velocity limiter of increased effectiveness.
Another object is a velocity control device of decreased weight.